When forming a well, such as an oil or natural gas well, a hole is drilled in the earth using an earth-penetrating drill bit situated at an end of one or more drilling tools which, in turn, are disposed at the end of numerous sections of pipe (i.e. a drilling assembly). The drilling assembly is then rotated to affect the drilling process. During or after the drilling process, it is advantageous to gather data and/or take measurements of the well being formed. The one or more drilling tools may include components that allow gathering of data and/or obtaining measurements regarding the well being drilled, the drilling tools being known generally as logging while drilling tools. Alternatively, such tools may be raised or lowered into a previously-drilled well to obtain data or measurements regarding the well.
The drill bit of the drilling assembly has a diameter larger than that of the drilling tools and pipe sections, so that when the drilling assembly bores into the earth, drilled earth can exit the hole from around the drilling assembly. To facilitate drilling and removal of the drilled earth from the hole, a drilling fluid (termed mud) is pumped through the drill bit during drilling. The mud mixes with the drilled earth as it flows through and exits the hole. As such, the mud includes dirt, rocks and other debris. The debris laden mud flows upward through the hole while the drilling assembly rotates during the drilling process. The components of the tools are thus constantly subjected to the damaging effects of debris hitting and abrading the components as a result of the flow of the debris-laden mud during the drilling process.
Tool components for well drilling need to be as rugged or robust as possible. One component typically used in well drilling tools is a pulse-echo acoustic transducer. Such transducers generate an acoustic pulse, which is reflected off the wall of the well, and returned to the transducer. By measuring the amount of time it takes for the reflected pulse to return to the transducer, and by analyzing other characteristics of the pulse, data about the well, such as the distance from the transducer to the well, can be determined. The signal and/or data derived from the signal is logged, often during the drilling process. Alternatively, transducers can be put into pre-drilled well bores to obtain data regarding the well.
Acoustic transducers may include a piezoelectric element mounted to a backing that is retained in a housing which is formed of a polymer such as PEEK (polyetheretherketone). For drilling applications, the housing is mounted in the drilling tool such that a face of the housing adjacent the piezoelectric element is exterior of the drilling tool. The exposed PEEK face is thus constantly subjected to the debris-laden mud flow during the drilling process, which can damage the face and cause it alter its acoustic properties over time, or to completely malfunction. An approach to address this issue could be to make the face thicker since a thicker face can inherently withstand more abrasion. However, it was determined that making a thicker face degrades the acoustic properties of the transmitted acoustic pulse and the received acoustic echo. Therefore, merely making the face of a pulse-echo acoustic transducer thicker does not create a more robust transducer.
Tests conducted by the inventors on pulse-echo piezoelectric acoustic transducers having PEEK faces of 0.100 inches (0.100″) thick and 0.300 inches (0.300″) thick exhibited undesirable qualities. Particularly, each thickness produced an acoustic signal with limited, bandwidth and exhibiting excessive ringdown.
While the forgoing has been described with reference to pulse-echo acoustic transducers used in drilling operations, and logging while drilling applications in particular, the same problems exist for pulse-echo acoustic transducers used in medical and other applications.